2 accounts on at the same time

ok a person i added to minecraft Realms [Gh0st_Girl] had someone come on the server with her name and she was already on the server.

What I expected to happen was...:
this not to happen
What actually happened was...:
2 [Gh0st_Girl] comming on the server

Steps to Reproduce:
1.i think some1 created the same username at the same time as her and it made 2 accounts with the same user name